This invention relates to a process for modifying particle size distribution. In a more specific aspect, this invention relates to a process for modifying the particle size distribution of fine-grained Tertiary kaolin clays. This invention also relates to the kaolin clay pigments which are produced by this process and to paper products which are filled and/or coated with these pigments.
For ease of reference, this invention will be described with regard to fine-grained Tertiary kaolin clays from the State of Georgia, U.S.A. However, this invention will be understood as applicable to fine-grained Tertiary kaolin clays from other geographical regions.
Kaolin is a naturally occurring, relatively fine, white clay which may be generally described as a hydrated aluminum silicate. Kaolin clay, after beneficiation, is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
The kaolin clays found in the State of Georgia can be generally classified into two types: hard kaolins and soft kaolins. The hard kaolins are fine-grained (usually finer than 80% less than 2 microns e.s.d.) and typically found in Tertiary age sediments. The soft kaolins are relatively coarse-grained (usually coarser than 65% less than 2 microns e.s.d.) and typically found in Cretaceous age sediments.
The Cretaceous soft kaolin clays of Georgia contain abundant stacks and books of kaolinite, together with individual platelets of kaolinite. The coarse-grained nature of these clays is due to the recrystallization of kaolinite during weathering and/or diagenesis of the kaolin clay deposit. Tertiary hard kaolin clays of Georgia contain individual kaolinite platelets with a face-to-face association characteristic of a sedimentational fabric.
Kaolin clay filler and coating pigments are conventionally produced by dispersing a kaolin clay in water with the appropriate dispersing agent (such as sodium hexametaphosphate or sodium silicate) and a pH modifier (such as sodium carbonate or sodium hydroxide). After dispersion, the kaolin slurry is degritted using hydroseparators and screens to remove very coarse ( greater than 325 mesh) particles which are predominately silica and mica impurities. Following degritting, modification of the particle size distribution is made using centrifugation to remove coarse kaolinite particles. This centrifugation classification is generally used to produce pigments which conform to industry standards for paper coating applications. For example: a No. 1 coating clay has approximately 90% less than 2 microns e.s.d., and a No. 2 coating clay has approximately 80% less than 2 microns e.s.d.).
Another particle size modification technique used in the production of kaolin pigments is delamination. The Cretaceous aged coarse-grained kaolin clays contain coarse stacks and books of kaolinite which, when delaminated, cleave upon the basal planes of the kaolinite particle to form very platy particles with a high aspect ratio (aspect ratio=particle length/particle thickness).
After classification, the kaolin slurry can be treated using a reducing agent such as sodium hydrosulfite, an acid such as sulfuric acid and alum such as aluminum sulfate. This reductive leaching process improves the brightness of the pigment by reducing the ferric iron phases to soluble ferrous iron. In some instances, other processing techniques are conducted prior to leaching to produce higher brightness products. These techniques include flotation, selective flocculation and magnetic separation.
Following reduction of the iron, the clay slurry can be dewatered using filtration to remove the soluble iron along with other soluble salts. The dewatered kaolin clay is then redispersed using a dispersing agent such as sodium polyacrylate and a pH modifier such as sodium carbonate. After redispersion, the clay slurry may be spray dried or made into a high-solids slurry (65-72% solids by weight) for shipping.
One characteristic that kaolin clay pigments impart to paper coatings is sheet gloss. The relative fineness and particle shape of the kaolinite particles in the coating pigment control the sheet gloss, with the finer kaolinite forming a smoother and glossier paper surface. Furthermore, during the paper coating process, calendering may be used to further modify sheet gloss by passing the paper through a heated nip and compressing the coating. The advantage of using fine-grained clays over coarser clays is that fine-grained clays do not require a large amount of calendering (pressure or heat) which leads to a weaker sheet, reduced brightness, reduced opacity, slower production rates and overall higher paper processing costs.
The effect of fine kaolinite (i.e., particles less than 0.2 microns e.s.d.) on the printing characteristics of paper coatings is well known. The variation in the amount of fine kaolinite in paper coating pigments greatly affects the interaction of printing inks with the paper coating. In general, as fines increase, the print gloss of a coated paper decreases due to the effect of the fines on ink adsorption. Similarly, as fines decrease, the print gloss of a coated paper increases. Prior art has shown that the reduction of fines improves print gloss.
Modification of the particle size distribution of kaolin pigments used in paper and filler applications has been shown to alter the opacity of kaolin pigments. In general, a narrow particle size distribution improves the opacity of paper by increasing the light scattering ability of the pigment. Furthermore, delamination of coarse-grained Cretaceous kaolins which contain books and stacks of kaolinite can also modify the light scattering ability of the pigment by modifying particle shape and packing.
Many patents have described the use of kaolin pigments to provide improved sheet gloss, print gloss and opacity in paper coatings. In these patents, particle size modification, specifically by the removal of fine kaolinite (i.e.,  less than 0.2 microns e.s.d.), improves the paper coating and printing characteristics.
U.S. Pat. No. 2,992,936 describes a coating clay with improved brightness, gloss and opacity by producing a pigment with a narrow particle size distribution. U.S. Pat. No. 3,085,894 describes the same clay; however, this patent describes blends of the kaolin clay and calcium carbonate.
In general, the particle size distribution providing these improved characteristics in the prior art is as follows:
88-100% by wt. less than 1.7 microns e.s.d.
85-97% by wt. less than 1.5 microns e.s.d.
70-84% by wt. less than 1.0 microns e.s.d.
25-37% by wt. less than 0.5 microns e.s.d.
10-15% by wt. less than 0.3 microns e.s.d.
U.S. Pat. No. 4,279,661 describes grinding and/or classification to produce a mineral filler or coating composition with the particle size distribution having at most 15% less than 0.2 microns e.s.d. and 80-95% less than 1.0 micron e.s.d. This pigment is stated to provide a higher sheet and print gloss over those pigments with particle size distributions outside the claimed distribution.
U.S. Pat. No. 4,943,324 describes a  greater than 89 G.E. brightness pigment with a particle size distribution in which at most 15% of the particles are less than 0.3 microns e.s.d. The process uses fines removal (i.e., xe2x80x9cdefiningxe2x80x9d) and delamination to produce this pigment. Furthermore, the defined and delaminated clay is chemically treated to produce the pigment with improved opacifying properties.
U.S. Pat. No. 4,948,664 describes a coating composition comprising a clay with not more than 5% of the particles less than 0.25 microns e.s.d. and at least 5% of the particles having a particle size not less than 10 microns e.s.d. This patent further discloses that at least 40% of the particles have a particle size not less than 3 microns e.s.d. This composition is disclosed as useful in providing an improved pigment for use in gravure printing.
U.S. Pat. No. 5,085,707 describes paper coating compositions utilizing a defined and delaminated pigment, and these compositions cover a variety of combinations using standard delaminated clay, No. 1 and No. 2 coating clays and the pigment as described in the patent. Both chemically treated and untreated versions of the defined pigment are described. Furthermore, the patent discloses that the defined and delaminated pigment should contain 70% of its particles between 2.0 and 0.3 microns e.s.d. Compositions utilizing this pigment are stated to have increased opacifying and gloss development properties compared to standard kaolin pigments.
U.S. Pat. No. 5,168,083 describes a process for producing a pigment with improved opacifying power which utilizes coarse-grained Cretaceous kaolins. As in many of the previous patents, the pigment contains 5-15% less than 0.2 microns e.s.d. This patent also states that xe2x80x9cscrub grindingxe2x80x9d is necessary to produce the pigment, and the patent discloses chemical treatment of the clay.
U.S. Pat. No. 5,169,443 describes an improved delaminated pigment having at most 12% less than 0.2 microns e.s.d. U.S. Pat. No. 5,411,587 also describes a pigment with this particle size distribution.
U.S. Pat. No. 5,645,635 describes the production of pigments using removal of fines and delamination. The particle size distribution of the product contains less than 5% less than 0.2 microns e.s.d. U.S. Pat. No. 5,749,958 describes a similar pigment but with a particle size distribution having less than 10% less than 0.2 microns e.s.d. In both patents, the pigments are stated as having improved opacifying characteristics in paper coatings.
According to the prior art, the particle size modification needed to produce pigments with improved paper coating characteristics has required that the amount of fines ( less than 0.2 microns e.s.d.) be reduced to extremely low levels, generally less than 15%. Furthermore, many of the prior art pigments require delamination or grinding to produce a pigment with improved paper coating properties. Finally, the clay material generally used in the prior art is a coarse-grained kaolin containing stacks and books of kaolinite which respond to a delamination process.
Due to the limitations of the prior art processes, there is a need in the industry for a process to modify the particle size distribution of fine-grained Tertiary kaolin clays without lowering the amount of kaolinite fines to prior art levels and without the need for a delamination process step.
Briefly described, the present invention provides for modifying the particle size distribution of a fine-grained Tertiary kaolin clay by a process which xe2x80x9ctailorsxe2x80x9d the kaolin clay by removing fine particles and coarse particles.
As used in this application, the term xe2x80x9ctailorsxe2x80x9d or xe2x80x9ctailoredxe2x80x9d refers to the process steps which remove fine particles and coarse particles from the kaolin clay to produce a pigment having a narrow particle size distribution.
In addition, as used in this application, and as measured by a Micromeritics SediGraph instrument, the term xe2x80x9cfine particlesxe2x80x9d refers to particles having an equivalent spherical diameter (e.s.d.) less than 0.2 microns, and the term xe2x80x9ccoarse particlesxe2x80x9d refers to particles having an e.s.d. greater than 2.0 microns.
This invention relates to the modification of the particle size distribution of a fine-grained Tertiary kaolin clay to improve the characteristics of a paper coated with this modified clay. The particle size modification involves the removal of coarse and fine kaolinite particles to produce a tailored clay pigment with a narrow particle size distribution.
The tailored pigments of the present invention exhibit improved paper coating characteristics (i.e., sheet gloss, print gloss and opacity) over standard kaolin pigments. Furthermore, these tailored pigments are superior to other tailored clay pigments in that improvements in paper coating characteristics are gained without substantially reducing the amount of fine kaolinite particles as required when producing tailored pigments using coarse-grained Cretaceous kaolins.
Accordingly, an object of this invention is to provide a process for modifying particle size distribution.
Another object of this invention is to provide a process for modifying the particle size distribution of fine-grained Tertiary kaolin clays.
Another object of this invention is to provide a process for modifying the particle size distribution of fine-grained Tertiary kaolin clays by tailoring the clays to remove fine particles and coarse particles.
Another object of this invention is to provide a process for tailoring fine-grained Tertiary kaolin clays by removing fine particles and coarse particles to produce a kaolin clay pigment with a narrow particle size distribution.
Another object of this invention is to provide a tailored kaolin clay pigment having a narrow particle size distribution.
Still another object of this invention is to provide a tailored fine-grained Tertiary kaolin clay pigment.
Still another object of this invention is to provide a tailored fine-grained Tertiary kaolin clay pigment which will provide improved opacity, sheet gloss and print gloss to a paper coated with this pigment.
Still another object of this invention is to provide a paper sheet coated with a tailored kaolin clay pigment having a narrow particle size distribution.
Still another object of this invention is to provide a paper sheet coated with a tailored fine-grained Tertiary kaolin clay and having improved opacity, sheet gloss and print gloss.